This invention relates generally to respiration systems, and, more particularly, to respiration systems in which a preselected volume of air or other breathable gas mixture is delivered to a patient during each breathing cycle.
Typically, the gas is delivered to the patient from a cylinder which contains a movable piston, and the volume of gas delivered from one end of the cylinder is measured by detecting the displacement of the piston in the cylinder. The piston is usually driven by air under pressure admitted at the opposite end of the cylinder and supplied from a separate pneumatic system. When the desired volume of gas has been delivered from the cylinder, the supply of air from the driving pneumatic system is shut off to terminate piston movement. Before the next inspiration phase of the patient breathing cycle, the piston is permitted to return to its initial position, and a new supply of breathable gas is admitted to the cylinder.
In order to minimize the frictional resistance to movement of the piston in the cylinder, either a bellows construction or a rolling seal is used to seal the piston to the cylinder. A rolling seal is basically a folded cylindrical sleeve of resilient material affixed circumferentially to both the cylinder and the piston, and having sufficient length to allow full movement of the piston in the cylinder.
With both the bellows and rolling seal designs, the flexibility of the sealing members introduces the possibility of significant error in the volume of gas delivered to the patient, since, when pneumatic driving pressure is admitted below the piston, the flexibility of the sealing member will cause some gas to be delivered to the patient before movement of the piston begins. Furthermore, there is always a time lag from the time that driving pneumatic pressure is admitted to the cylinder until the time that the piston begins moving and gas is delivered to the patient. Accordingly, there is a need in the respiration art for a volume-limited delivery means which overcomes the aforementioned problems, and the present invention fulfills this need.